The present invention relates to mechanical or chemical mechanical polishing of conductive layers.
Chemical mechanical polishing (CMP) have been used to pattern metal layers in fabrication of semiconductor integrated circuits. FIGS. 1 and 2 illustrate cross sections of a wafer in one fabrication process. The fabrication starts with a semiconductor substrate 120. MOS transistor source/drain regions 130 are formed in substrate 120, and a transistor gate 140 is formed over a channel region extending between the source/drain regions. (Other circuit elements may also be present) A dielectric 150 is formed over the substrate 120 and patterned as needed. A conductive layer 160 (e.g. aluminum) is formed on dielectric 51 and suitably patterned. A dielectric 170 is formed on layer 160. Trenches 184 are etched in dielectric 170 to define interconnect lines. Vias 190 are etched at the bottom of the trenches at selected locations to expose the layer 160. Then a metal layer 194 (tungsten or copper) is deposited over the structure, filling the trenches 184 and the vias 190.
The wafer is polished by CMP (FIG. 2) until the metal 194 is removed from the top surface of dielectric 170. Trenches 184 and vias 190 remain filled with metal 194, providing he interconnect lines contacting the layer 160 in vias 190.
To ensure complete removal of metal 194 from the top surface of dielectric 170, the wafer is overpolished, i.e. the polishing continues for some time after the dielectric 170 is exposed. When the dielectric has been exposed, the polishing proceeds faster in a region 210 having a high density of metal lines 194, than in a surrounding region 220 having no metal lines. Consequently, the top surface of region 210 is indented (xe2x80x9cerodedxe2x80x9d). The erosion undesirably changes the electrical properties of interconnect lines 194. Also, the top surface of the wafer becomes non-planar, which complicates fabrication of overlying layers. See U.S. Pat. No. 6,340,602 issued Jan. 22, 2002 to Johnson et al. and incorporated herein by reference.
In order to ensure that the erosion does not exceed some acceptable limit, the polished wafers are examined to determine the maximum erosion value xcex94. If xcex94 exceeds the limit, the wafer is discarded. Also, if xcex94 approaches or exceeds the limit, the CMP process is adjusted to reduced the erosion in subsequent wafers.
The maximum erosion xcex94 can be measured with a probe tool 230, e.g. a stylus profilometer or a scanning probe microscope (e.g. atomic force microscope). Tool 230 has a probe 240, a circuitry 250 for processing the data from the probe, and a mechanism (not shown) for moving the probe relative to the wafer. See e.g. U.S. patent application publication No. 2001/0047682 published Dec. 6, 2001. Probe 240 contacts the wafer top surface, or comes very close to the wafer to sense the top surface topography. Undesirably, the wafer can be contaminated by the probe. Therefore, more expensive and complicated optical instruments have been used instead of the probe instruments to measure the erosion in production wafers.
The invention is defined by the appended claims which are incorporated into this section by reference. This section summarizes below some features of the invention.
Some embodiments of the invention make it safer to use a probe for the erosion measurements on production wafers. This will now be illustrated with reference to FIG. 3, showing the top view of a polished wafer. In this example, trenches 184 and metal lines 194 form a periodic pattern in region 210. The pattern has a pitch P defined as a distance between similar points on the adjacent metal lines. P=W+S, where W is the width of each metal line and S is the distance between the adjacent lines 194. It is well known that the erosion increases with the W/P ratio.
The inventor has studied the dependence of the erosion on the size of regions 210, and has discovered that if the W and S parameters are held constant, then the size dependence is weak. Consequently, the erosion in a large region 210 can be estimated by measuring the erosion of a smaller test structure. For example, a test structure of 50xc3x9750 xcexcm can be incorporated into the wafer and used to monitor the erosion in a region 210 having dimensions on the order of several millimeters (e.g. the lines 194 can be bitlines or strap lines of a memory array). The erosion of the test structure can be measured with a probe tool 230 since damage to the test structure is acceptable. The probe may contact the wafer, or come as close as 2 xcexcm to the wafer, or as close as 1 xcexcm, or closer. xcex94test structure of 50xc3x9750 xcexcm can be formed on a scribe line or a margin of the wafer.
In some embodiments, the test structure has the same layers (e.g. 150, 160, etc.) as the actual circuitry region 210 In other embodiments, some layers (e.g. 140 or 160) are omitted in the test structure. Also, the test structure may have additional layers.
The invention is not limited to the embodiments described above. The invention is not limited to rectangular regions 210 or periodic structures. For non-periodic structures, the pitch P is defined as the distance between the corresponding points on the adjacent metal lines (e.g. between the left edges of the adjacent lines), and the pitch P may vary over the structure. Also, the invention is not limited to use of a probe to measure the erosion. Some aspects of the invention consist in the presence of certain structures on the wafer and are not limited to the use of a probe. The erosion of the wafers with such structures can be measured with optical instruments without a probe. Other features of the invention are described below. The invention is defined by the appended claims.